This invention relates to the protection of receptors sensitive to light in the visible and infrared (IR) regions of the electromagnetic spectrum, including but not limited to human eyes and optical detectors and, more specifically, to optical limiters which allow passage of normal ambient light levels such as sunlight but prevent passage of high levels of light such as laser light, and which are comprised of substantially transparent material having a dopant that is photo-reactive.
Visible and IR light receptors are capable of detecting electromagnetic radiation (light) at various intensity levels and at various wavelengths (colors) in the spectral region from approximately 400 nm to 5000 nm. Examples of such light receptors include but are not limited to the human eye and optical detectors/sensors which produce a response (thermal, electrical etc.) whenever illuminated.
The human eye, optical detectors/sensors and photo-receptors can be damaged by exposure to high intensities of light. For example, optical detectors can be exposed beyond their capabilities and destroyed by either continuous or short duration exposure to a laser beam. Similarly, the retina of the eye can be damaged by being exposed to a laser beam for only a brief period of time. In effect, the retina and the nerves within the retina are burned by the intense light of the laser beam.
In laboratory and industrial settings, protective glasses or goggles are commonly worn to protect the eyes from exposure to laser light. These eye protection devices are based on filtering techniques and, typically, are made from tinted or colored materials which may be selected for protection over specific wavelength ranges of laser light. Because of the coloration of these materials, light at wavelengths other than that of the laser light are attenuated. This restricts a broader portion of the spectrum, significantly altering the spectral features of the transmitted light. As a result, these protective glasses or shields typically have a disadvantage that the visual perception of the colors of objects and images observed are either altered or obscured. For example, goggles suitable for protection against green laser light at 532 nm, a frequency doubled Nd:YAG emission, or at 514.5 nm, an argon ion laser emission, are typically orange colored. This does not allow the wearer to perceive colors in the blue or green region of the visible spectrum.
While not all environments of usage require unaltered spectral transmission, there are numerous environments which have strict requirements for substantially unaltered spectral transmission. One specific example of such an environment is within an airplane cockpit. Many aircraft, both military and civilian, are using color coded instrumentation to display information for the pilot. Additionally, light displays projected on the wind screen or canopy and known as xe2x80x9cheads-upxe2x80x9d displays are color images which, if optically blocked, filtered or altered, may not be fully visible to the pilot.
The usage of lasers capable of blinding the cockpit crew of an aircraft has become a hazard for both military and commercial air traffic. It is therefore necessary to protect pilots"" eyes from exposure to intense laser radiation. It is not feasible to block the transmission of all wavelengths of light commonly emitted by lasers using existing filtering techniques inasmuch as the ability of the pilot to observe both instrumentation inside the cockpit and other objects outside the cockpit would be so altered as to render the flight crew partially blinded or severely limited.
Photo-reactive optical limiters such as sun glasses that change color with higher visible light spectrum intensity are commercially available. The reaction time of the photo-reactive elements is quite slow, on the order of 1-90 seconds, and is definitely not fast enough to provide any protection from laser beams which may be of a pulsed or continuous wave type. Thus, a laser beam impinging on the eye through an eyeglass lens of this type will not be attenuated sufficiently or quickly enough to prevent damage to the eye.
A protective device capable of protecting the human eye or optical detectors from laser radiation must have a very fast photo-reactive response and yet must be of sufficient transparency in the spectral region applicable to a given light receptor to allow it to function. In the case of visual perception, the protective device to be effective must also permit, without distortion, observation of color images as advantageously used in certain environments requiring protection from exposure to laser radiation.
It is an object of the invention to protect photo-receptors, including but not limited to a human""s eyes, and optical detectors from damage by exposure to high intensity lasers by changing the transmission characteristics of a transparent inorganic material interposed between the photo-receptor and the laser.
It is another object of the invention to alter the transmissibility of a transparent inorganic material by doping the inorganic material with photo-reactive ions to absorb or block the visible light in response to illumination of the doped material with high intensity laser radiation.
It is a further object of the invention to protect photo-receptors from the effects of laser light by altering the transmissiblity of a transparent inorganic material having a photo-reactive dopant in response to laser light causing excited state absorption, photoionization, color center formulation, charge transfer and/or combinations of these processes.
Many inorganic materials are transparent to visible and IR light. This transparency can be altered by the addition of dopant ions, either singly or in combinations. By adding dopants selected from the group of transition metal ions (elements with atomic numbers between 22 and 30), with the dopant levels in the range from one (1) percent to twenty (20) percent by weight for each dopant, the material containing these ions can become opaque to specific wavelengths of light whenever exposed to high intensity laser radiation. The transmission characteristics of such doped materials are initially determined by the host lattice and the ground state (lowest energy state) absorption bands of the dopant ions. Exposure of the material to laser light can induce new absorption bands in the material, referred to as photo-induced absorption (PIA). If the new absorption bands occur at the same wavelength as the laser light, the transmission of the laser light is substantially reduced or limited.
There are numerous processes in doped inorganic materials that have the capability of limiting the transmission of intense laser radiation including excited state absorption (ESA), photo-ionization, color center formation, charge transfer and trapping, and combinations of these processes. For example, at an intensity sufficient to cause damage to eyes or photo-detectors/sensors, light incident on the material is absorbed by the dopant ions that are originally in the ground state, thereby exciting the ions to an intermediate higher energy state.
Ions in this intermediate energy state have substantially stronger absorption characteristics than the ions in the ground state and can further absorb light exciting the ions to even higher energy states. This process is referred to as excited state absorption. The reduction in transmission due to this process is significant when a sufficient number of ions have been excited to the intermediate level. As a result, low intensities of light pass through the material substantially unaltered while the transmission of high intensity laser light is significantly reduced. The opaque region is confined to the volume of the material which has been exposed to the high intensity laser radiation.
The transmission of high intensity laser light can also be limited by charge transfer and trapping processes. In these types of processes, dopant ions in the material interact with the laser light which excites the ions to a higher energy state. Excitation of the ions to this higher energy state may occur through direct absorption of a photon of sufficient energy, or through a variety of other mechanisms including ESA or two photon absorption. Ions in this higher energy state interact with the host lattice or other dopant ions resulting in the release of an electron by the excited dopant ions. If an ion was originally in an N+ valance state, this process increases the valance state of the ions to (N+1)+ and is typically referred to as photo-ionization. The electron released by the excited dopant ion can recombine with the ion, become trapped in the lattice creating a color center, or combine with some other dopant ion in the material changing its valance state (for example from M+ to (Mxe2x88x921)+). Recombination of the electron with the photo-ionized dopant ion results in recovery of the material to its original transmission state. Formation of color centers or the transfer of the electron to another dopant ion changes the absorption characteristics of the material and can result in optical limiting. In this case, the transmission of light through the material is determined by the absorption characteristics of the dopant ion in the (N+1)+ valance state, the color centers and/or the new valance state of ions that have captured the electron. If the absorption bands induced by the laser light occur in the same wavelength region as the laser, these processes will reduce/limit the transmission of the laser light.
These excited state absorption, photo-ionization and charge transfer processes are reversible, allowing for recovery of the material transmission characteristics to the original state. For the case of ESA, the material spontaneously recovers through decay of the electron from the final excited state to the ground state. This occurs by spontaneous emission or by energy transfer to the host lattice (generation of phonons which result in the generation of heat). For the case of photo-ionization and subsequent charge transfer, the material can spontaneously recover upon recombination of the electron with the photo-ionized dopant ion or optical transmission can be restored by supplying energy to the material. For example, it may be possible to heat the material providing thermal energy to the electron to free it from the trap in the lattice, allowing it to reduce the photo-ionized dopant ions to their original valance state. This reversal process may also be accomplished by absorption of light (photons) by the trapped electron, freeing the electron from the trap and allowing recombination with the photo-ionized dopant ion.
The materials used to demonstrate this invention were single crystals doped with transition metal ions grown using conventional Czochralski crystal growth methods.
Crystals of yttrium orthoaluminate (YAlO3), yttrium aluminum garnet (Y3Al5O12), commonly referred to as YAG), and gadolinium gallium garnet (Gd3Ga5O12), commonly referred to as GGG) doped with manganese (Mn), combinations of Mn and cobalt (Co), or combinations of Mn and calcium (Ca) were shown to exhibit ESA and charge transfer and trapping which resulted in the limiting of the transmission of laser light at selected wavelengths.
These doped crystalline materials were substantially transparent in the visible and IR regions of the spectrum. In the visible spectral region these crystals had slight or light color casts which depended on the specific combination of host and dopant ions as well as the valance state of the dopant ions.
The response time within which the optical limiting process occurred is less than 0.5 ns and the degree of optical limiting was intensity dependent, a very advantageous characteristic for eye protection.
A more complete understanding of the invention may be had from the accompanying drawings and the detailed description to follow.